The present invention relates to a method and a device for welding two pieces, abutted in an assembling position, by means of a welding beam. While mainly developed for laser beam welding, this technology also may be appropriate for other spot welding processes, such as the plasma torch, TIG, MIG, MAG, or electron beam processes.
1. Field of the Invention
The invention particularly relates to the welding of tube lengths for realizing hydrocarbon transportation pipes, in particular on sea bottoms. This welding can take place in difficult conditions, such as is the case on a barge being subjected to movements induced by the sea, and to many vibrations generated by the equipment and motors located on board.
2. Description of the Related Art
For this purpose, a welding machine has already been conceived, comprising a fixed laser source which is spaced away from the tube lengths to be welded and the beam of which is guided, with the help of articulated optical path means, out to a laser head which bears a focussing device and is fixed on a support table rotating around the axis of the tube lengths, which are kept abutted by their ends. For the welding, the support table is rotated by means of the first electric motor around the tube lengths, while the laser head position is adjusted by an axial translation by means of a second electric motor. Such a device is described in the French Patent 93 04642 published under the reference number 2 704 166.
Laser welding requires an extremely precise positioning of the laser beam focussed onto the joint to be welded, namely +/-0.2 mm. The joint tracking control usually needs using a sensor fixed on the laser head, for detecting the eddy currents. This joint tracking system guides the laser head but not the laser beam itself. As a result, any misalignment of the optical path induces a deviation between the laser beam position and the joint to be welded, which may cause a defective welding since the laser beam kinematics cannot be simply deduced from the laser head kinematics. The eddy current sensor furthermore does not allow detecting the joint in an abutting assembly without clearance.
The object of the present invention is to realize a direct and precise piloting of the welding beam and of its impact by servo-controlling its position, where the welding plasma is formed, at the position of the joint to be welded.
For this purpose, the invention concerns a method for laser beam welding two pieces abutted in an assembling position along an assembly joint, to be welded by means of a welding head with a spot shaped beam, characterized in that at least during the welding phase a joint is continuously observed by means of a camera mounted integral with the welding head, by moving the welding head and camera assembly along the joint and by sensing the orthogonal position of the joint for each position of the welding head and camera assembly along the joint, in that the impact position of the welding beam is continuously observed, and in that a piloting of the configuration of an optical path for guiding said beam is insured by servo-controlling the impact position at the position of a joint detected by controlling a translation of the welding head and cameral assembly orthogonally to the joint.
The camera particularly may consist of either a standard camera, or more particularly a CCD camera or an infrared camera for the laser beam position finding operation.
A tracking of the welding plasma is thus performed by servo-controlling its position on the joint as a function of the detected joint position; in other words, the laser beam itself, and not just the laser head, is positioned in real time onto the joint to be welded. This allows reaching for the laser beam, and consequently for the created welding plasma, a positioning precision which is sufficient to insure a good weld, as opposed to the state of the art.
In an embodiment of the invention, during a learning phase, the welding head and camera assembly is moved along the entire assembly joint and a map of the orthogonal positions of the joint is established with respect to a linear marker arranged parallel to the joint line and, during a later welding phase, the welding is performed during a second movement of the welding head and camera assembly along the joint while servo-controlling the welding plasma position when the welding beam is being operated.
Advantageously, a linear optical marker extending along the assembly joint consequently is fixed onto one of the two pieces being maintained in the assembling position, in order to establish the map of the distance between this marker and the joint line for all operating positions of the welding head along the joint. Controlling the plasma position adjustment with respect to the marker is then all that is needed for insuring that the orthogonal distance between the joint and the marker, as supplied by the map, is respected. It should be noted that this servo-controlled piloting may be performed in real time, during the same welding head movement along the joint.
In particular, using the marker allows, during the learning phase and the welding phase, full insensitivity vis-a-vis the possible movements of the tube lengths with respect to the welding head and camera assembly. A rectilinear edge marker furthermore facilitates the position finding operation for the joint position tracking.
In the case where both pieces to be assembled are abutted tube lengths, the marker preferably consists of collar shaped band, the positioning of the welding head and camera assembly is obtained by an axial movement with respect to the tube lengths axis, and both map completion and welding are obtained by an orbital motion around the tube lengths. It is useful for such a collar to have an axial mark, i.e. along a generator of the pieces, which determines by reference an initial position of the welding head and camera assembly during the orbital motion.
In the case where both pieces to be welded are abutted, snugly fitted flat surfaces, it might be useful to chamfer the edge of one of the pieces. It then is easy to sense the joint position by observing its shadow under a lighting beam.
For the map completion, several movements of the welding head and camera assembly may be performed. A complete and precise map is thus obtained after several learning phases in cases where some points of the map are blurred or do not exist.
During the welding phase, an anti-dazzle screen and a filter are advantageously provided between the camera and the welding plasma. This allows obtaining a correct vision of the marker during this welding phase, and attenuating and filtering the plasma for more precisely sensing of the latter.
According to another characteristic of the invention, the optical observation beam of the camera is divided into two spatially spaced parts, which respectively are output towards the vicinity of the marker on the one hand and the joint or the welding plasma on the other hand. This allows using a low field objective, affording a good precision, even in cases where the marker and the joint to be welded or the plasma are separated by a large distance.
The invention also concerns a device for implementing the above defined method. Advantageously, the camera includes a CCD type sensor, located upstream of the laser head with respect to the movement along the assembly joint, so as to leave the desirable room for the laser head downstream. It further is advantageous for the welding head to include a focussing device with a beam reflection mirror rotatably mounted around its optical axis.
In cases where the invention is applied to a device designed for welding abutted tube lengths and including a fixed laser source spaced away from the tube lengths to be welded, the welding beam advantageously is guided by means of an articulated optical path out to a laser head including a focussing device fixed on a support table rotating around the axis of the tube lengths maintained in abutment at their ends. Such an optical path preferably includes, starting from the laser source, a first part the means of which are floatingly mounted with all degrees of freedom on a fixed support table, and a following second part leading the beam out to the focussing device which is fixed on a rotating support table, rotatable around the tube lengths with respect to the fixed support table.
Such an arrangement allows obtaining a "soft" optical path between the laser source and the focussing device. The relative motions between the fixed support table and the tube lengths to be welded (in a case where the fixed support table is integrally assembled with the tube lengths) are entirely governed by the first floating part of the optical path, and the tracking of the marker or the joint is performed on the second part of the optical path without any need for taking into account the relative motions of the various elements of the welding device. As previously indicated, these movements indeed are quite numerous when a welding is performed aboard a barge.
This floating mounting may notably be realized by means of a ball bearing located on the fixed support table and supporting a freely translatable device in the direction perpendicular to the fixed support table. It also is possible to use a sliding linking, based for instance on polytetrafluoroethylene shoes or an air cushion device, or else a movement support table along x-y coordinates with rotation.
Advantageously, the first part of the optical path includes an arm mounted in a freely articulated manner with respect to an axis parallel to the fixed support table, so as to obtain a variable slanting of said arm with respect to the fixed support table. This first part of the optical path may include an arm both ends of which are rotatably mounted with respect to one another around the axis of the arm, which allows obtaining one of the degrees of freedom, here in rotation.